def isPrime(a):
        if a == 1: return False
        test = True
        k = a - 1
        while k > 1:
            if not a % k:
                test = False
                break
            k -= 1
        return test    
n = input()
r = []
if len(str(n)) == 4:
    for i in range(len(n)):
        for k in range(i + 1,len(n) + 1):
            a = int(n[i:k])
            if isPrime(a):
                r += [a]   
print(max(r) if r else 0)
